Beijing yesterday announced an official investigation into former security chief Zhou Yongkang, shattering the decades-old political taboo of not prosecuting the highest ranking Communist Party officials for corruption.

Zhou, who retired from the Politburo Standing Committee in 2012, is being probed for "serious disciplinary violations" - a euphemism for corruption. His case will be handled by the Communist Central Commission for Discipline Inspection, Xinhua said in a terse report.

The announcement ended intense speculation since the South China Morning Post exclusively reported last August that current and retired party leaders had agreed to open a case against Zhou. He is the most senior Chinese official brought down by corruption in modern history and the case will have a far-reaching impact.

It ends the unspoken rule agreed by party elders towards the end of the Cultural Revolution that Politburo Standing Committee members would not be prosecuted on the grounds of economic or social crimes - for the sake of the party's unity. It will also galvanise President Xi Jinping in his fight against corruption, which he has identified as the single biggest threat to China and the Communist Party.

It breaks the convention that the PSC members are untouchable

STEVE TSANG, CHINA POLICY INSTITUTE

"This is a big deal," said Professor Steve Tsang, director of the China Policy Institute at the University of Nottingham in Britain. "It breaks the convention that Politburo Standing Committee members are untouchable for criminal transgressions."

Other analysts and party sources said the announcement was intended to reduce behind-the-scenes struggles among top power holders and reunite the party under one banner ahead of a key gathering next month.

"The announcement suggests leaders have finally agreed on major policy issues, which will be finalised at the summer summit at Beidaihe [resort]," said Zhang Ming, a professor of political science at Renmin University. The Beidaihe summit is traditionally held in August.

On the same day, the leadership announced a key meeting for all party elite to be held in October to decide policy direction. President Xi said yesterday the rule of law would top the agenda at this meeting, the fourth plenary of the 18th Party Congress.

Given that Zhou had been in charge of law and order before his retirement, the public announcement of his corruption case will give Xi tremendous political credibility to reform the outdated legal regime.

The Post reported in May that the top leadership was eager to convince the party elite that the case against Zhou was a step to reinforce the rule of law, not only an internal political struggle.

Shortly after the Xinhua announcement, People's Daily website published a commentary saying that Zhou would not be "the last tiger" to be investigated for corruption.

"Some corrupted party members hold tremendous power in their hands. Their roots are deep and connections wide. Fighting them is not easy. But we must do it. Otherwise the people will not go with us," it said.

Watch: China's Xi cementing power with probe into Zhou: analyst

Experts believed Zhou, like his key party ally Bo Xilai, would eventually face public prosecution.

Their downfalls are possibly linked. Zhou was said to be a political patron of Bo, the disgraced former Chongqing party boss. The two had allegedly teamed up in a failed attempt to grab power. Bo is now serving life in prison for corruption.

"I believe Zhou is likely to be tried in public just like Bo," said Zhang. "And he will receive a harsher penalty of a suspended death sentence."

Du Guang, a retired professor at the Central Party School, said it would be difficult to expose all the alleged crimes that Zhou had committed because "it would bring a much bigger crisis for the party". He agreed Zhou's case was likely to be heard in public .

A person connected with the party inner circle said Zhou had tried to meet former president Jiang Zemin , who was seen as his political patron.

"But Jiang turned down all the requests."

The news was picked up by news outlets around the world. Media sources said the mainland press was told to limit coverage.

Additional reporting by Cary Huang

Watch: Video of Zhou Yongkang at an NPC meeting in 2012

This article appeared in the South China Morning Post print edition as: